The present invention relates to an apparatus for skidding structures along a surface, and more particularly to an apparatus for skidding a heavy skid base and substructure for a drilling rig on an offshore platform to position the drilling equipment over a particular well.
It is common in drilling wells offshore that numerous wells will be drilled from the same platform. A system is required to move the drilling rig to the next well in a series of wells to be drilled or worked. In the past, this has been accomplished by skidding a skid base, to which the drilling rig is attached, along the top of the drilling platform.
The two I-beams comprising the top chord of the deck truss of the platform typically serve as the skidding surface for a skid base which spans across the beams. This beam is sometimes referred to as the skid beam or capping beam. The skid base is pushed or pulled longitudinally across the platform by two skidding assemblies. For lateral movement of the rig, a substructure carrying the drilling equipment sits on and skids on the top flanges of the skid base. Generally, two skidding assmeblies push or pull this substructure across the top of the skid base.
In the prior art, there are two basic types of rig skidding assemblies. The first is the rack and pinion type. The second is the rod and piston type.
The present invention falls in the second class of rig skidding systems. This type of system employs, for example, a hydraulic cylinder having one end connected to the skid base and the opposite end connected to a device for engaging the capping beams of the platform. By extending or retracting the cylinder rod, the skid base is moved.
It is known in the prior art to provide a hydraulic gripper to anchor the end of the hydraulic cylinder to the capping beam. In this system, parallel plates are disposed above and below the top flange of the capping beam on both sides of the web. The plates are hydraulically squeezed together to grip the top flange of the beam so the hydraulic cylinder may be activated to move the skid base with respect to the platform.
In another prior system, the hydraulic cylinder is anchored to the capping beam by a claw. The claw includes a plate which slides on the beam. The capping beam has a plurality of rows of equally spaced rectangular openings, called jacking holes, through the top flange of the beam on both sides of the web. The bottom of the claw typcially has four L-shaped fingers attached which fit through four rectangular openings in the beam and under the flange to anchor the end of the cylinder.
In yet another prior art system, the end of the hydraulic cylinder is pinned to a claw or shoe which rides on the capping beam. The shoe has guides attached extending downwardly on either side of the top flange of the beam and inwardly to prevent lifting of the shoe from the beam. A sleeve attached to the shoe contains a circular pin which moves down through the shoe to engage the round holes in the capping beam. The pin is mounted on the lateral center line of the shoe and accordingly a row of spaced apart holes must be cut through the top flange above and generally into the web along the centerline of the capping beam.
A major problem with the last two systems is the cutting of openings through the capping beams. These beams are usually major structural members of the platform and cutting holes through them reduces their structural strength. It is obviously preferable that no holes be cut through such structural members, although some platform designers may make allowance for the loss of steel caused thereby. In the case of the system last described, the holes extend down into the web of the I-beam; a very undesirable situation.
In older platforms the jacking holes may already be cut through the capping beam. If this is the case, the drilling contractor must either modify his claw to fit the holes or the oil company must modify the holes or the hole pattern to fit the contractor's claw.
Other prior art of which Applicant is aware is that prior art cited in application Ser. No. 313,166, now abandoned.